Tests: Use real "Event" instead of mocking HistoryChange

The problem

Apparently JSDom doe snot support HistoryChange events. It thus uses a mock function: https://github.com/PrivateBin/PrivateBin/blob/846476402fe933493d1edacc3f200b2343e3f57e/js/test/UiHelper.js#L22

It's generally a bad idea to have methods in your prod code just for mocking/testing purposes.

The solution

similar to how the Clipboard event is mocked/created (just as a custom Event that bubbles), this event should be similarly mockable, should not it?

Alternatives

Keep as is.

Additional context